Growth in Length at the Epiphyseal Growth Plate

Zones of Growth

• Resting (Cartilage) Zone

  • No chondroblast activity present.
  • Function: Chondrocytes anchor the epiphysis to the epiphyseal plate.

• Proliferating (Cartilage) Zone

  • Characterized by high chondroblast activity.
  • New cartilage tissue is formed as new chondrocytes replace old chondrocytes on the diaphysis side of this zone.

• Hypertrophic Zone

  • Involves the enlargement of old chondrocytes and ultimately leads to their death, creating spaces within the cartilage.

• Calcification Zone

  • Features osteoclasts that remove old, dead chondrocytes.
  • Blood vessels invade the area, bringing calcium ions (Ca²⁺) crucial for new bone formation.

• Ossification Zone

  • Characterized by high osteoblast activity, where osteoblasts secrete osteoid.
  • The osteoid mixes with calcium (Ca²⁺) to form new bone tissue.
  • Result: The diaphysis pushes upward on the epiphyseal plate, contributing to overall bone growth.

Embryonic Ossification and Bone Growth

Types of Ossification

1. Intramembranous Ossification

   • Forms flat bones, notably:
     • Skull (except mandible)
     • Clavicles

2. Endochondral Ossification

   • Utilizes a hyaline cartilage model to form:
     • All other bones in the body
   • Key Structures:
     • Epiphysis
     • Epiphyseal Plate
     • Diaphysis

Epiphyseal Plate Closure

• At the end of puberty, estrogen surges lead to:
  • Stopping of proliferation in Zone 2, which results in the closure of the epiphyseal plate.
  • Final Result: Diaphysis bone tissue seals to epiphyseal bone tissue, creating a scar known as the "Epiphyseal Line."

Bone Growth in Thickness - Appositional Growth

• Involves several processes:

1. Periosteal Cells Activity

   • Periosteal cells on the bone surface differentiate into osteoblasts that secrete osteoid.
   • Osteoid mixes with calcium (Ca²⁺) to form bone, resulting in the formation of bone ridges that encapsulate blood vessels.

2. Endosteal Surface Activity

   • Some osteoclast activity occurs on the endosteal surface of the medullary cavity, leading to the widening of this cavity.

3. Balance of Activities

   • Osteoblast activity on the external surface outpaces osteoclast activity on the medullary cavity.
   • Outcome: Bone grows in diameter while the diaphysis becomes thicker, enhancing compact bone thickness and widening the medullary cavity.

Factors Affecting Bone Growth

1) Minerals

• Essential minerals needed in diet for proper bone growth:
  • Calcium, magnesium, potassium, etc.

2) Vitamins

• Vitamin A

  • Stimulates osteoblast activity.

• Vitamin C

  • Necessary for collagen synthesis.

• Vitamin D

  • Enhances absorption of calcium (Ca²⁺) in the small intestine.

• Vitamin K and B

  • Required for synthesis of bone proteins.

3) Hormones

• Growth Hormone (GH)

  • Secreted by the anterior pituitary gland, plays a crucial role in childhood bone growth.
  • Stimulates liver to secrete Insulin-like Growth Factors (IGFs) which promote chondroblast activity at the epiphyseal plate.

• Estrogens and androgens

  • Become the most influential during puberty, causing significant "growth spurts."
  • Function: These hormones drive closure of epiphyseal plates after puberty.

• In adults

  • Steroid sex hormones influence bone resorption and promote new bone until middle age (particularly in women after menopause) where this activity decreases significantly.